Engineering Tools

Snap-Fit Design Calculator

Design a cantilever snap-fit that clicks without cracking. Enter the beam geometry and material, and check the bending strain against the material’s allowable — then get the deflection force and the mating (assembly) force.

Snap-fit inputs

Straight cantilever beam with constant rectangular cross-section — the most common snap-fit style. Dimensions in mm.

Material

Assembly frequency

Beam width b (mm)

Thickness at root t (mm)

Beam length L (mm)

Deflection (undercut) y (mm)

Lead angle α (deg)

Friction coefficient μ

Reference

How the numbers are derived

Standard cantilever snap-fit equations as published in the classic BASF and Bayer snap-fit design guides.

Equations & data

ε	Max bending strain at the root = 1.5·t·y / L² (constant rectangular section). Must stay below the material allowable.
y_max	Max permissible deflection = ε_allow·L² / (1.5·t). If your undercut is bigger, lengthen the beam, thin the root, or taper it.
P	Deflection force = b·t²·E·ε / (6·L) — the force to bend the beam through y.
W	Mating force = P·(μ + tanα) / (1 − μ·tanα) — the push force felt during assembly.
ε_allow	Typical one-time-assembly allowables: ABS 6%, PC 4%, PC/ABS 5%, POM 6%, PA66 (conditioned) 8%, PP 8%. Use ~60% of these for parts snapped repeatedly.

Watch-outs

The root radius matters as much as the math — a sharp inside corner concentrates strain and cracks first. Use r ≥ 0.5·t.
Use flexural modulus from the material datasheet at your service temperature; glass-filled grades have far lower allowable strain than unfilled.
A 90° return angle makes the joint permanent; smaller return angles allow release — compute retention force with the return angle in the W equation.
Weld lines, gate location, and fiber orientation can locally halve the strain a molded beam survives.
These equations assume the mating part is rigid; if both parts flex, the effective deflection is shared.

Designing plastic parts that have to click — and last?

Snap-fits fail in the details: root radii, weld lines, creep, tolerance stack on the undercut. If your enclosure or mechanism depends on molded features working the thousandth time as well as the first, bring the design to a consultation.

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